1. Field of the Invention
This invention relates generally to an 8-phase Phase-Shift Keying (8 PSK) demodulator and more specifically to such a demodulator featuring an improved function by which resynchronization can be achieved in a shorter time duration as compared with known 8 PSK demodulators. This invention is highly suited for use in a digital radio transmission system.
2. Description of the Prior Art
A digital radio transmission system is susceptible to multipath fading or the like and invites waveform distortion of the transmitted signal, which degrades signal quality and which may cause a short break in transmission reception. In order to minimize these problems, it is the current practice to employ a transversal equalizer using a transversal filter.
The transversal equalizer in a digital radio transmission system, however, has encountered the difficulty that distortion of the transmitted signal is apt to exceed the equalizing capability thereof. More specifically upon the distortion reaching a level at asynchronism of clock and carrier signals in a demodulator is induced and results in asynchronism of the control loop of the transversal filter. These conditions induce signal distortion within the equalizer itself. Therefore, even if the distortion of the transmitted signal again falls within the capability of the equalizer, the synchronized state is not automatically restored in the equalizer A known approach to solving this problem is to reset the tap gain signal generator coupled to the transversal filter to its initial state upon the occurrence of asynchronism in the demodulator This prior art approach maintains the equalizer at reset until resynchronism of the clock and carrier in the demodulator occurs. Accordingly, as the equalizer remains inoperative during this time period, the control loop of the equalizer is not brought into synchronization unless the waveform distortion of the transmitted signal is lowered to a considerable extent.
It is therefore highly desirable to have a transversal equalizer restored to its normal state as quickly as possible upon the distortion of the transmitted signal being reduced to a level at which the equalizer is able to perform its function To this end, intermittent resetting of a transversal filter has been proposed in U.S. Pat. No. 4,567,599 assigned to the same entity as the instant invention. According to this prior art, when asynchronism is detected in a demodulator, a reset signal is intermittently generated to render the transversal filter operative at intervals during a period in which the transversal filter is paused. This prior art strives to shorten the inoperative duration of the equalizer by discontinuously checking quality recovery of an incoming IF signal through intermittent resetting of the equalizer However, such a negative approach has proven insufficient to effectively shorten the inoperative period of the equalizer.
Further, no 8 PSK demodulator has yet been proposed which is provided with the above-mentioned function plus automatic gain control for maintaining the incoming IF signal to a constant level.